Coastal Change Hazards: Hurricanes and Extreme Storms

Category-4 Hurricane Katrina moving towards the coasts of Louisiana, Mississippi, and Alabama on August 28, 2005. (photo credit: NASA Terra Satellite) [larger version]

Extreme Storms

Overview

Three types of storms are studied by the USGS Extreme Storms research group, including hurricanes, northeasters, and storms generated during El Niño conditions.

Hurricanes form over warm, tropical ocean waters when surface temperatures exceed 80° F. Hurricane winds exceed 74 mph and circulate in a counter-clockwise motion in the Northern Hemisphere.

Northeasters (also known as Nor'easters) are winter storms that form in the mid-latitudes between the months of September and April. These storms move northward in the Atlantic Ocean, paralleling the coastline of the eastern United States and Canada.

El Niño is a phenomenon associated with unusually warm surface waters in the tropical and equatorial regions of the eastern Pacific Ocean. El Niño events have major implications on global weather, including an increase in storms along the western coast of the United States.

Strong winds build large waves and storm surge in Ft. Lauderdale, FL during Hurricane Ernesto in late August 2006. (photo credit: Susan Caplan) [larger version]

Although each type of storm is unique, there are several factors common to all storm types. These factors include strong winds, the generation of large waves, and elevated water levels known as storm surge.

• These storms are all characterized by strong winds that blow over the surface of the ocean. Winds are known to cause structural damage to coastal property, but are also important because of their ability to generate waves and storm surge.
  
  
• Surface winds generate large waves that can travel thousands of miles across the ocean, until they eventually reach the coastline. When large waves attack the shore, they can cause beach erosion, cliff failure, and the destruction of coastal property.
  
  
• Winds generated by these storms pile up water along the coastline, creating higher than normal water levels known as storm surge. Storm surge elevations have been known to exceed 20 feet along the open coast during the most extreme storms, submerging beaches and flooding coastal properties. An increase in coastal water level also allows waves to advance on portions of the coastline normally well beyond the zone of wave attack.

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